The present disclosure relates to a method for driving a storage element that has plural magnetic layers and performs recording by utilizing spin torque magnetization reversal, and a storage device.
Along with dramatic development of various kinds of information apparatus ranging from mobile terminals to high-capacity servers, further enhancement in the performance, such as increases in the degree of integration and the speed and power consumption reduction, is pursued also regarding elements such as memory and logic that configure the information apparatus. In particular, the advance of the semiconductor non-volatile memory is significant and the spread of the flash memory as a large-size file memory is progressing at such a rapid pace as to drive out the hard disc drive. On the other hand, in anticipation of expansion into the code storage and the working memory, development of ferroelectric random access memory (FeRAM), magnetic random access memory (MRAM), phase-change random access memory (PCRAM), etc. is being advanced to replace NOR flash memory, DRAM, etc., which are generally used presently. Part of these memories has been already put into practical use.
In particular, the MRAM is capable of high-speed and almost-infinite (1015 times or more) rewriting because data is stored based on the magnetization direction of a magnetic body. The MRAM has been already used in the fields of the industrial automation, the airplane, etc. Because of its high-speed operation and high reliability, the MRAM is expected to be expanded into the code storage and the working memory in the future. However, it still has challenges in power consumption reduction and capacity increase in practice. They are fundamental challenges attributed to the principle of recording in the MRAM, i.e. the system in which magnetization reversal is caused by a current magnetic field generated from an interconnect.
As one method to solve this problem, studies are being made on a system of recording, i.e. magnetization reversal, that is not based on the current magnetic field. Particularly, researches relating to spin torque magnetization reversal are active. A storage element by the spin torque magnetization reversal is configured with magnetic tunnel junction (MTJ) as with the MRAM. This configuration utilizes a characteristic that spin-polarized electrons passing through a magnetic layer pinned to a certain direction give torque to another magnetic layer that is free (whose direction is not fixed) when entering this magnetic layer. In this configuration, the flow of a current equal to or larger than a certain threshold value causes reversal in the free magnetic layer. Rewriting of 0/1 is performed by changing the polarity of the current. The absolute value of the current for this reversal is equal to or smaller than 1 mA in an element with scale of about 0.1 μm. In addition, scaling is possible because this current value decreases in proportion to the element volume. Moreover, this system does not require the word line for generating the current magnetic field for recording, which is necessary for the MRAM, and therefore also has an advantage that the cell structure may be simpler.
Hereinafter, the MRAM utilizing the spin torque magnetization reversal will be referred to as the ST-MRAM (spin torque-magnetic random access memory). Great expectations are placed on the ST-MRAM as a non-volatile memory that enables power consumption reduction and capacity increase while keeping the MRAM's advantages that high-speed operation is possible and the number of times of rewriting is almost infinite.
However, in the ST-MRAM, voltage is applied to the MTJ also in recording differently from the related-art MRAM. This voltage in recording is higher than the read voltage. Therefore, the possibility of the occurrence of the electrical breakdown of the MTJ (attributed mainly to e.g. the dielectric breakdown of a thin tunnel barrier) is higher compared with the MRAM. That is, to ensure high rewriting endurance equivalent to that of the related-art MRAM in the ST-MRAM, it is important to pay attention to the electrical breakdown of the MTJ and take measures to avoid it.
As one of the measures, there is proposed e.g. a method of suppressing the deterioration of an insulator by applying a reverse-polarity voltage (refer to e.g. Nakano et. al., ECS Trans. 19(2), 711 (hereinafter, Non Patent Document 1)). This technique aims at suppressing a resistance decrease at the tunnel junction due to electric field application by applying a pulse voltage of the reverse polarity.
It is inferred that two processes, i.e. a reversible process and an irreversible process, exist in the dielectric breakdown in a rough classification (refer to e.g. P. S. Ku et. al Proc. of 44th Annual international Reliability Physics Symposium, p. 437 (hereinafter, Non Patent Document 2)). The reversible process is equivalent to that an annihilable defect is generated due to an electric field or coupling between atoms forming the insulator is deformed in a restorable range due to an electric field. This annihilation or restoration of the defect is realized by aging, heat treatment, etc. On the other hand, the irreversible process is equivalent to that coupling between atoms is broken into a restoration-impossible state due to an electric field, and thus recovery is impossible in this process.